It is standard practice in the industrial production of glass sheets, whether simple window glass, structural elements, mirrors, or the like, to roughly cut the sheets slightly oversize, and then to grind-trim them down to exact dimensions. For this grinding the sheet is supported on a table with its sheet edges overhanging the table above a template having a template edge geometrically congruent to, and normally of the same dimensions as, the shape the sheet is to have after grinding. A grinder radially engages the edge of the sheet with a feeler in engagement with the template edge so that as the table and template are rotated about a vertical axis or the grinder is orbited about the table, the grinder grinds the rough-cut sheet on the table down to a size corresponding exactly to that of the template.
In order to speed this operation a system described in German Patent No. 46,419 uses a pair of oppositely effective grinders. Thus opposite edges of the workpiece can be ground simultaneously so that the grinding operation itself is twice as fast. Such an arrangement requires that the workpiece be turned through 90.degree. if it has four edges to be ground.
It has also been suggested to use a pair of diametrally opposite grinders that radially oppositely engage the sheet carried on a rotary grinding table. In such an arrangement it is only necessary to rotate the table through 180.degree. to grind all its edges. Nonetheless, since each grinding operation rotates the table and template through 180.degree., it is of course necessary to reposition the table before depositing a new sheet on it, unless of course the sheets are symmetrical about a point. Accordingly the table must be turned back to its starting position after each such grinding operation in order to receive a new workpiece. Such a return step slows production.
As a rule the rough-cut sheets are simply stacked up in a supply station adjacent the grinding station. A carriage horizontally displaceable on overhead tracks is provided with lifters, normally of the vacuum or suction type, that are dropped down onto the topmost sheet of the stack and lifted to raise it so this sheet can be transported horizontally and deposited on the work table. Such an arrangement is described in German patent document No. 2,756,443.
Unless the rough-cut sheets are exactly positioned in the supply station and the transporter functions with great precision, normally necessitating very slow action to prevent the picked-up sheet from slipping laterally on the transporter, it is necessary for an operator in the grinding station to position the sheets exactly with respect to the template, that is position them in vertical registration above the template. If the workpiece is laterally offset it will be ground down too much on one edge and not at all on the opposite edge.